Prostaglandin E2 regulates Foxo activity via the Akt pathway: implications for pancreatic islet beta cell dysfunction
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Prostaglandin E2 (PGE2) is a well-recognised inhibitor of glucose-stimulated insulin secretion (GSIS). The aim of this study was to investigate the signalling pathway of PGE2 in beta cell function regulation in HIT-T15 cells and isolated rat islets.
Materials and methods
mRNA levels of the prostaglandin E receptor 3 (Ptger3) were measured by real-time PCR. Western blot analysis was used to detect changes in the levels of PTGER3, phosphorylated and total Akt, phosphorylated and total forkhead box ‘Other’ (Foxo). Transient transfection and reporter assays were used to measure Foxo transcriptional activity. The biological significance of PGE2 in beta cell function was analysed using MTT, flow cytometry and GSIS assays.
We found that treating HIT-T15 cells with exogenous PGE2 stimulated Ptger3 gene expression specifically, and diminished cAMP generation. These were accompanied by the downregulation of Akt and Foxo phosphorylation in HIT-T15 cells and isolated rat islets. Moreover, PGE2 upregulated basal and partially reversed constitutively active Akt-inactivated Foxo transcriptional activity. Furthermore, GSIS was impaired in PGE2-treated HIT-T15 cells and isolated islets. However, the dosage used in the above experiments did not affect beta cell viability and apoptosis. In addition, insulin-like growth factor 1 (IGF-1) pretreatment reversed the effects of PGE2, and wortmannin treatment abolished the preventive effects of IGF-1.
Our observations strongly suggest that PGE2 can induce pancreatic beta cell dysfunction through the induction of Ptger3 gene expression and inhibition of Akt/Foxo phosphorylation without impacting beta cell viability. These results shed light on the mechanisms of PGE2 actions in pancreatic beta cell dysfunction.
KeywordsAkt Foxo GSIS Islets Pancreatic beta cell PGE2
constitutively active Akt
fetal bovine serum
forkhead transcription factor Foxo1
forkhead transcription factor Foxo3a
forkhead box ‘Other’
glucose-stimulated insulin secretion
insulin-like growth factor-1
prostaglandin E receptor
Roswell Park Memorial Institute-1640
Types 1 and 2 diabetes mellitus are characterised by autoimmune destruction and functional impairment of insulin-secreting beta cells in the pancreatic islets of Langerhans [1, 2, 3]. Although the initial events leading to the development of diabetes mellitus are not well characterised, proinflammatory prostaglandins, including prostaglandin E2 (PGE2) appear to play an important role. Three isoforms of cyclooxygenase (COX) have been characterised to date, including two constitutive subtypes, COX-1 and COX-3, and one inducible isoform, COX-2 . In contrast to most mammalian cells, beta cells constitutively and dominantly produce the COX-2 isoform of PGE2-generating enzymes rather than COX-1 . Prior studies have demonstrated that PGE2 inhibits glucose-stimulated insulin secretion (GSIS) in clonal beta cells and isolated islets [6, 7], and that selective inhibition of COX-2 attenuates the development of diabetes in the low-dose streptozotocin mouse model and protects rat islets from cytokine-induced inhibition of GSIS [8, 9]. Despite the established role of PGE2 in pancreatic beta cells, the exact molecular mechanisms of PGE2-mediated inhibition of insulin secretion remain poorly understood.
PGE2 exerts its actions on cells by interacting with one or more of its four G protein-coupled prostaglandin E receptor (PTGER) subtypes named PTGER1, PTGER2, PTGER3 and PTGER4, all of which are coupled to signal transduction systems that involve phosphoinositide hydrolysis, calcium or adenylate cyclase activity. However, only PTGER3 has been shown to have post-receptor activities that result in a decrease in cAMP levels [10, 11].
Akt, also known as protein kinase B, is a serine–threonine kinase that is activated by phosphatidylinositol 3-kinase (PI3K) at Thr308 and/or Ser473. Accumulating evidence indicates that upon activation in response to many different growth factors, hormones and external stresses, Akt serves as a pivotal regulator of glucose transport, glycolysis, protein production, lipogenesis, glycogen synthesis, suppression of gluconeogenesis, cell survival, determination of cell size and cell-cycle progression as well as insulin synthesis and secretion [12, 13, 14, 15]. However, the precise role of Akt in the beta cell dysfunction associated with diabetes mellitus remains controversial [15, 16].
One downstream target of the PI3K/Akt pathway that could mediate the PGE2 effects is the forkhead box ‘Other’ (Foxo) class of transcription factors, a subfamily of the large group of forkhead transcription factors. Mammalian cells contain three members of this family, Foxo1 (FKHR), Foxo3a (FKHRL1) and Foxo4, whose functions are blocked by Akt via the phosphorylation of three conserved residues, which leads to their sequestration in the cytoplasm away from target genes. However, dephosphorylation of Foxo transcription factors leads to nuclear entry and modulates their targeted gene expression [17, 18]. There is increasing evidence that Foxo transcription factors play an important role in mediating the effects of hormone and growth factors on diverse physiological functions, including cell proliferation, apoptosis, metabolism and insulin synthesis [14, 19]. Nevertheless, the exact role of Foxo in beta cell dysfunction is ambiguous.
In the present study, we chose to test the hypothesis that Akt and Foxo are involved in PGE2-mediated pancreatic islet beta cell dysfunction. Using the glucose-responsive beta cell line HIT-T15 and isolated rat islets, we found that PGE2 can induce pancreatic beta cell dysfunction through the induction of Ptger3 gene expression and the upregulation of Foxo activity in a PI3K/Akt pathway-dependent manner without affecting beta cell viability.
Materials and methods
Roswell Park Memorial Institute-1640 (RPMI-1640) medium, glucose-free DMEM and the Lipofectamine Plus transfection kit were obtained from Invitrogen Life Technologies (Grand Island, NY, USA). FBS was purchased from Hyclone (Logan, UT, USA). PGE2 and wortmannin were purchased from Sigma Aldrich (St Louis, MO, USA). Human recombinant insulin-like growth factor-1 (IGF-1) was manufactured by R&D Systems (Minneapolis, MN, USA). Rabbit polyclonal antibody against PTGER3 was purchased from Cayman Chemical (Ann Arbor, MI, USA). Rabbit polyclonal antibodies against Ser256-phosphorylated FKHR, Thr24-phosphorylated FKHRL1, Thr308-phosphorylated Akt, Ser473-phosphorylated Akt, total FKHR and total Akt were purchased from Cell Signaling Technology (New England Biolabs, Beverly, MA, USA). Horseradish peroxidase-conjugated anti-rabbit IgG was obtained from Amersham Pharmacia Biotech (Piscataway, NJ, USA). The Detergent Compatible (DC) Protein Assay kit was purchased from Bio-Rad Laboratories (Hercules, CA, USA). The RNeasy Mini Kit was from Qiagen (Hilden, Germany). The Luciferase Assay System was obtained from Promega (Madison, WI, USA). The TaqMan One-step PCR Master Mix Reagents kit and Assays-on-Demand gene expression products were purchased from ABI (Applied Biosystems, Foster City, CA, USA). The firefly luciferase reporter construct pGL3-FKHR (containing three FKHR-binding sites) was a kind gift from M. J. Anderson (La Jolla, CA, USA). The constitutively active Akt (CA-Akt) construct was kindly provided by J. Zieg (Boston, MA, USA).
HIT-T15 cells were kindly provided by R. P. Robertson (Seattle, WA, USA). The cells were grown in a humidified atmosphere containing 95% air and 5% CO2, and maintained in RPMI-1640 medium (11.1 mmol/l of glucose) supplemented with 10% FBS, as described previously . Before treatment, the cells were depleted in RPMI-1640 medium containing 0.5% BSA and 3 μg/ml indomethacin for 8 h. The cells were then washed in PBS, and the depletion medium was reintroduced. At that time, wortmannin and/or IGF-1 were added in certain experiments before the addition of PGE2. For all compounds prepared in alcohol or DMSO, the final concentration of alcohol and DMSO in the culture medium was kept less than 0.1%. Vehicle controls were prepared for all treatments.
Islet isolation and culture
All animal studies were performed according to guidelines established by the Research Animal Care Committee of Nanjing Medical University, China. Male Sprague–Dawley rats (250–300 g, purchased from Shanghai Laboratory Animal Centre, Chinese Academy of Sciences, Shanghai, China) were used. Islet isolation and culturing techniques have been described previously . Freshly isolated islets were transferred to sterile six-well dishes and cultured in DMEM containing 11.1 mmol/l glucose supplemented with 10% FBS, 10 mmol/l HEPES, 100 U/ml penicillin and 100 μg/ml streptomycin. The islets were allowed to equilibrate for 3 h, at which point they were counted and re-picked into static incubation tubes (ten islets per tube) and cultured overnight at 37°C. The next morning, the islets were depleted in DMEM containing 0.5% BSA and 3 μg/ml indomethacin for 4 h. Then the islets were washed in PBS, and depletion medium with the corresponding combination of wortmannin (300 nmol/l), IGF-1 (100 ng/ml) or PGE2 (1 μmol/l) was reintroduced. GSIS studies were performed 24 h later. For western blot analysis, aliquots of about 600 islets were transferred into six-well dishes and cultured overnight in DMEM as described above. The next morning, the islets were depleted in DMEM containing 0.5% BSA and 3 μg/ml indomethacin for 4 h. Then the islets were treated without (control) or with PGE2 (1 μmol/l) in the depletion medium for 0.5, 1, 2, 4 and 8 h, after which they were collected and lysed.
Real-time PCR assay
Cells were cultured and treated with PGE2 as described above. The total RNA samples were extracted from HIT-T15 cells treated without (control) or with PGE2 (1 μmol/l) for 0.5, 1, 2 and 4 h using Qiagen RNeasy Mini Kits. A TaqMan ABI Prism 7000 Sequence Detection System (Applied Biosystems) was used for the analysis. One-step real-time PCR was conducted with a TaqMan One-step PCR Master Mix Reagents kit. The TaqMan probes and primers were acquired by Assays-on-Demand (Applied Biosystems). The primer and probe sequences are given in Electronic supplementary material (ESM) Table 1. All data were analysed using the values of the β-actin gene levels as a reference.
Western blot analysis
HIT-T15 cells and isolated rat islets were cultured and treated as described above, and lysed with ice-cold lysis buffer containing: 50 mmol/l Tris–HCl, pH 7.4; 1% NP-40; 150 mmol/l NaCl; 1 mmol/l EDTA; 1 mmol/l phenylmethylsulfonyl fluoride; and complete proteinase inhibitor mixture (one tablet per 10 ml; Roche Molecular Biochemicals, Indianapolis, IN, USA). After protein content determination using a DC Protein Assay kit, western blotting was performed as described .
cAMP determination assay
Cells were harvested by centrifugation at 2,000 g for 10 min, and 0.1% trichloroacetic acid in 95% ethanol was added to the cell pellets. After 0.5 h extraction, the supernatants were recovered by centrifugation at 5,000 g for 10 min and dried. The cAMP levels were determined by RIA. Protein pellets were assayed using a DC Protein Assay kit to normalise the cAMP concentrations.
Transient transfection and luciferase reporter assay
FKHR transcriptional activity was assessed in HIT-T15 cells using the FKHR luciferase reporter construct pGL3-FKHR. We used a plasmid containing the β-galactosidase gene driven by the cytomegalovirus promoter (Clontech Laboratories, Palo Alto, CA, USA) as an internal control. The HIT-T15 cells grown in 24-well dishes were cotransfected with two (pGL3-FKHR and β-galactosidase) or three plasmids (CA-Akt, pGL3-FKHR and β-galactosidase) using the Lipofectamine Plus transfection kit, according to the manufacturer’s instructions. Twenty-four hours after transfection, the cells were cultured in serum-free RPMI-1640 medium containing 0.5% BSA for 4 h before treatment. The cells were then gently washed in PBS, and the depletion medium was reintroduced. At that time, wortmannin (300 nmol/l) was added to the medium in certain experiments 0.5 h before the addition of IGF-1 (100 ng/ml) and PGE2 (1 μmol/l). Then cells were incubated for an additional 12 h, and harvested for luciferase reporter assays. Luciferase activity was measured with a luminometer (TD-20/20; Turner Designs, CA, USA) using the Luciferase Assay System. β-Galactosidase activity was detected to normalise any variations in the transfection efficiency.
Cell viability was determined using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays. Briefly, the cells were seeded in 96-well dishes at 1 × 104 to 2 × 104 cells per well, and treated without (control) or with different concentrations of PGE2, as described above, for 24 h. Then each well was supplemented with 10 μl MTT (Sigma Aldrich) and incubated for 4 h at 37°C. The medium was then removed, and 150 μl DMSO (Sigma Aldrich) were added to solubilise the MTT formazan. The optical density was read at 570 nm.
Flow cytometry analysis
HIT-T15 cells (1.5 × 106 cells per well) were cultured in six-well dishes and treated without (control) or with 1, 2, 5, 10 μmol/l PGE2 for 24 h. The cells of each well were then harvested and fixed with 1 ml 75% ice-cold ethanol at −20°C overnight. After fixation, the cells were washed in PBS and stained with 500 μl propidium iodide solution (50 μg/ml in PBS) containing 25 μg/ml RNase. The cells were incubated at room temperature for 0.5 h in the dark, and analysed using a FACSCalibur flow cytometer and Cellquest Pro software (Becton Dickinson Immunocytometry Systems, San Jose, CA, USA) for data acquisition and analysis.
HIT-T15 cells (5 × 105 cells per well) were seeded into 1 ml RPMI-1640 medium with standard glucose concentration (11.1 mmol/l) in 12-well dishes, and treated with corresponding drugs for 24 h as described above. Following preincubation for 1 h in glucose-free RPMI-1640 medium and drug solutions, the cells were treated for 1 h in RPMI-1640 medium and drug solutions with low (0.2 mmol/l) or stimulatory (11.1 mmol/l) glucose concentrations . Isolated rat islets were cultured and treated as described above. Then the islets were preincubated for 1 h in glucose-free DMEM with the appropriate drug combinations, and treated for 1 h in DMEM containing drug solutions and basal (3 mmol/l) or stimulatory (17 mmol/l) concentrations of glucose. After the static incubation, the supernatants were obtained and frozen at −70°C for subsequent insulin concentration determination. The insulin levels were measured using RIA as described previously , or a rat/mouse insulin ELISA kit (Linco Research, St Louis, MO, USA).
Comparisons were performed using Student’s t test between two groups, or ANOVA in multiple groups. Results are presented as means±SEM. A p value of less than 0.05 was considered to be statistically significant.
PGE2 stimulates Ptger3 gene expression and protein production and decreases cAMP synthesis in HIT-T15 cells
PGE2 decreases phosphorylation of Akt in HIT-T15 cells and isolated rat islets, which is reversed by IGF-1
PGE2 decreases phosphorylation of Foxo in HIT-T15 cells and isolated rat islets, which is reversed by IGF-1
The effects of PGE2 on FKHR transcriptional activity
To examine whether the effect of PGE2 on FKHR transcriptional activity is mediated by Akt, we transiently cotransfected HIT-T15 cells with or without the CA-Akt expression plasmid and pGL3-FKHR (all cells were transfected with β-galactosidase plasmid simultaneously as an internal control). Consistent with previous results [17, 18], FKHR luciferase activity was decreased significantly (by 46% vs control, p < 0.01; Fig. 4b) by CA-Akt expression. However, the addition of PGE2 (1 μmol/l) to the CA-Akt-cotransfected cells partially reversed the inhibitory effect of CA-Akt on the FKHR luciferase reporter activity (98% increase, p < 0.01; Fig. 4b).
To further examine the relationship between reduced Akt activity and PGE2 effects on FKHR transcriptional activity, we treated the transfected cells with wortmannin, IGF-1 and PGE2, as described in Materials and methods. IGF-1 attenuated the stimulatory effects of PGE2 on FKHR luciferase activity (p < 0.01; Fig. 4c). Furthermore, wortmannin, a well-known PI3K inhibitor, reversed the attenuation effect of IGF-1 (p < 0.01; Fig. 4c).
The effects of PGE2 on HIT-T15 cell viability and apoptosis
To confirm that the PGE2 dose (1 μmol/l) used in this study did not induce any obvious impairment of cell survival, we assessed the effects of PGE2 on apoptosis and cell-cycle progression using flow cytometry analysis. Consistent with our MTT assay results, we observed no significant apoptosis or cell-cycle phase alteration in HIT-T15 cells treated with PGE2 in the range of 1–10 μmol/l (Fig. 5b), suggesting that PGE2 induces beta cell dysfunction without affecting cell survival.
PGE2 inhibits GSIS via the PI3K/Akt signalling pathway
Accumulating evidence indicates that Akt plays a central role in the regulation of glucose transport, glycolysis, protein production, lipogenesis, glycogen synthesis, suppression of gluconeogenesis, cell survival, determination of cell size and cell-cycle progression . Many reports have also demonstrated that Akt activation plays an important role in promoting pancreatic beta cell survival and preserving beta cell function [16, 28]. In this study, we demonstrated that PGE2 decreased the levels of Ser473- and Thr308-phosphorylated Akt in both HIT-T15 cells and isolated rat islets. Moreover, IGF-1 reversed the inhibitory effects of PGE2 (1 μmol/l) on GSIS in HIT-T15 cells and isolated rat islets. Meanwhile wortmannin, a PI3K inhibitor, abolished the protective effect of IGF-1. These data suggest that suppression of the PI3K/Akt pathway is involved in PGE2-induced beta cell dysfunction. Importantly, our demonstration using MTT and flow cytometry analysis that this concentration of PGE2 (1 μmol/l) did not affect beta cell viability indicates that PGE2 may exert its inhibitory effect on beta cells without affecting cell survival. Consistent with our observations, recent studies have revealed that reducing Akt activity in beta cells resulted in dysregulation of insulin secretion without affecting beta cell mass and development [13, 15], and Akt has also been shown to play a key role in insulin synthesis . Hence, we presume that dysregulation of insulin synthesis and secretion resulting from diminished Akt activity may account for PGE2-induced beta cell dysfunction, although the underlying mechanisms of this dysregulation have yet to be resolved.
Foxo transcription factors, a subfamily of the large group of forkhead transcription factors, are phosphorylated and regulated by Akt and play crucial roles in mediating the effects of insulin and growth factors on diverse physiological functions, including cell proliferation, apoptosis and metabolism [19, 29, 30, 31]. We found in this study that the levels of Ser256-phosphorylated FKHR and Thr24-phosphorylated FKHRL1 were decreased in response to PGE2 treatment. The effect of PGE2 on intracellular signalling was further investigated with a luciferase reporter gene system. We observed that PGE2 stimulated FKHR transcriptional activity markedly in pancreatic beta cells. The addition of exogenous PGE2 partially reversed CA-Akt-inactivated FKHR luciferase activity. Furthermore, IGF-1 attenuated the stimulatory effects of PGE2 on FKHR luciferase activity, while wortmannin reversed the attenuation effect of IGF-1. Collectively, these results further confirm the functional association of PI3K/Akt in PGE2-mediated beta cell dysfunction. Previous studies have shown that FKHR is a negative regulator of insulin synthesis that acts by decreasing PDX1 production . In addition to its important roles in the development and differentiation of pancreatic islets and in beta cell specific gene expression , PDX1, an important downstream target of Foxo transcription factors, functions as an essential mediator of the glucose effect on insulin gene expression on differentiated beta cells . In accord, our study demonstrated that PGE2 could dephosphorylate Foxo transcription factors, prompting them to enter the nucleus and modulate the expression of target genes. For example, Foxo could induce the downregulation and nucleocytoplasmic translocation of PDX1, resulting in a reduction of insulin expression . The precise mechanisms mediating this effect, however, remain to be elucidated; specifically further evidence is needed to confirm the role of this pathway in PGE2-induced beta cell dysfunction.
In conclusion, we report for the first time that PGE2 can induce pancreatic beta cell dysfunction through the induction of Ptger3 gene expression, inhibition of intracellular cAMP generation and upregulation of Foxo activity via suppression of the PI3K/Akt signalling pathway, without affecting beta cell viability. This finding is best illustrated in Fig. 7, which shows Akt and Foxo as key regulators in PGE2-mediated dysfunction in pancreatic beta cells. Our studies contribute to the understanding of the underlying mechanisms by which PGE2 regulates pancreatic beta cell function and provide important clues for intervention in the diabetes mellitus disease course.
The authors are grateful to M. J. Anderson and J. Zieg for providing plasmids and R. P. Robertson for the HIT-T15 cell line. This work was supported by grants from the National Natural Science Foundation of China (30370676) and the Special Funds for Major State Basic Research Program of China (973 Program, 2006CB503908) to X. Han.
Duality of interest
The authors declare that they have no duality of interest.